Ipsilateral block or can a single ECGbe used for the diagnosis?

Background: Pre-excitation syndrome (premature excitation of the ventricles) is a congenital anomaly in the structure of the cardiac conduction system and consists in the presence of an additional atrioventricular connection. The possibility to assume the presence of pre-excitation syndrome accompanied by tachycardia based on an electrocardiogram provides a clue to the correct diagnosis and subsequent treatment of the patient. Clinical case description: A clinical case of a 56-year-old patient is presented, who was admitted to the FRCC of the FMBA of Russia in January, 2021 with paroxysms of previously undiagnosed tachycardia. During Holter monitoring, an episode of heart palpitations was recorded. When analyzing an ECG fragment, it was possible, by calculating the tachycardia cycle length, to suspect the presence of a latent ventricular pre-excitation syndrome, which was accompanied by the development of orthodromic atrioventricular reciprocal tachycardia with a bundle branch block on the side of the additional atrioventricular connection (ipsilateral block). The patient underwent endocardial electrophysiological examination to confirm the presence of the bundle, followed by the catheter treatment of the atrioventricular connection. A good postoperative clinical result was obtained. Conclusion: It is important to be able to make a differential diagnosis between the presence of a latent pre-excitation syndrome with the development of orthodromic atrioventricular reciprocal tachycardia with the bundle branch block on the side of the extra atrioventricular junction (ipsilateral block) and other supraventricular tachycardias with an aberration along one of the bundle branches, in order to determine the tactics of the patient management and to control the effectiveness of the treatment.